Prepare the crust: Cube the butter into ¼-inch pieces and place them in the freezer. Combine the flour and salt in a food processor and pulse briefly. Add the chilled butter cubes and pulse again until the mixture resembles coarse crumbs. With the processor on, slowly add the ice water, a couple of tablespoons at a time, until the dough just starts to hold together. Divide the dough in two, shape each half into a disk about 4 to 5 inches wide, wrap in plastic, and chill for at least 30 minutes.

Preheat your oven to 375°F.

Make the filling: In a medium pot, bring the blueberries to a boil. Once boiling, reduce the heat and stir in the sugar, lemon juice, cinnamon, salt, and butter. Let it cook over medium-low heat for about 5 minutes. Remove from heat, stir in the tapioca, then return to low heat for 10 minutes, stirring now and then to break up any clumps. Take off the heat again, stir in vanilla, and allow the filling to cool for 20 to 25 minutes.

Roll and shape: On a floured surface, roll one dough disk until it’s about ⅛ inch thick. Use a large round cookie cutter to make circles. Brush the edges with the egg-and-water mixture.

Fill and seal: Place 2 to 3 tablespoons of the cooled filling in the center of each dough circle. Fold the dough over into a half-moon shape, pressing the edges with a fork to seal. Transfer to a parchment-lined baking sheet. Repeat with the remaining dough and filling. Chill the prepared pies on the sheet for 20 minutes.

Bake: Brush the tops of the chilled pies with the egg-and-cream wash. Bake on the middle rack for 15 minutes or until golden brown. Let them cool on a wire rack before serving.